U.S. patent application number 10/128752 was filed with the patent office on 2002-11-21 for radio communication system.
This patent application is currently assigned to KONINKLIJKE PHILIPS ELECTRONICS N.V.. Invention is credited to Baker, Matthew P.J., Hunt, Bernard, Moulsley, Timothy J..
Application Number | 20020173302 10/128752 |
Document ID | / |
Family ID | 26246003 |
Filed Date | 2002-11-21 |
United States Patent
Application |
20020173302 |
Kind Code |
A1 |
Baker, Matthew P.J. ; et
al. |
November 21, 2002 |
Radio communication system
Abstract
A radio communication system comprises a primary station (100)
and at least two secondary stations (110a, 110b), each of which
comprises at least one antenna (108, 118). The primary station is
arranged to transmit respective communication channels to each of
the secondary stations from respective mutually exclusive subsets
of its antennas (108). The communication channels are defined
solely by the respective subsets of antennas (108), and each
secondary station has means (414) for extracting from the received
signals the subset comprising its respective communication
channel.
Inventors: |
Baker, Matthew P.J.;
(Canterbury, GB) ; Moulsley, Timothy J.;
(Caterham, GB) ; Hunt, Bernard; (Redhill,
GB) |
Correspondence
Address: |
Corporate Patent Counsel
U.S. Philips Corporation
580 White Plains Road
Tarrytown
NY
10591
US
|
Assignee: |
KONINKLIJKE PHILIPS ELECTRONICS
N.V.
|
Family ID: |
26246003 |
Appl. No.: |
10/128752 |
Filed: |
April 23, 2002 |
Current U.S.
Class: |
455/422.1 ;
455/560 |
Current CPC
Class: |
H04B 7/04 20130101 |
Class at
Publication: |
455/422 ;
455/560 |
International
Class: |
H04Q 007/20 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 25, 2001 |
GB |
0110125.2 |
Sep 21, 2001 |
GB |
0122745.3 |
Claims
1. A radio communication system comprising a primary station having
at least two antennas and at least two secondary stations each
having at least one antenna, wherein the primary station comprises
means for transmitting respective communication channels between
the primary station and first and second secondary stations from
respective mutually exclusive subsets of the primary station's
antennas, wherein the first and second secondary stations are in
the coverage area of the primary station antennas in both subsets,
wherein the respective channels are distinguished solely by the
respective subsets of antennas, and wherein the first and second
secondary stations each comprise means for estimating from the
received signals the subset of signals comprising its respective
communication channel.
2. A radio communication system as claimed in claim 1,
characterised in that one of the subsets of the primary station's
antennas comprises at least two antennas and in that multiple input
multiple output techniques are used in transmissions from this
subset to its respective secondary station.
3. A radio communication system as claimed in claim 1 characterised
in that the system comprises a further communication channel
between the primary station and one of the secondary stations,
which channel is not solely distinguished by a subset of antennas
from which it is transmitted.
4. A primary station having at least two antennas for use in a
radio communication system having at least two secondary stations,
wherein means are provided for transmitting respective
communication channels between the primary station and first and
second secondary stations from respective mutually exclusive
subsets of the primary station's antennas, wherein the first and
second secondary stations are in the coverage area of the primary
station antennas in both subsets, and wherein the respective
channels are distinguished solely by the respective subsets of
antennas.
5. A primary station as claimed in claim 4, characterised in that
means are provided for transmitting known signals from each of the
antennas comprised in the subsets, thereby enabling a secondary
station to characterise the radio channels between the primary
station antennas and the secondary station antennas.
6. A primary station as claimed in claim 4, characterised in that
means are provided for receiving signals from the secondary
stations providing information on the quality of received signals
and for making use of the information to determine which antennas
to assign to each subset.
7. A secondary station having at least one antenna for use in a
radio communication system having respective communication channels
between a primary station having at least two antennas, the
secondary station and a further secondary station, wherein the
communication channels are transmitted from respective mutually
exclusive subsets of the primary station's antennas, wherein the
secondary station and the further secondary station are in the
coverage area of the primary station antennas in both subsets,
wherein the respective channels are distinguished solely by the
respective subsets of antennas, and wherein means are provided for
estimating from the received signals the subset of signals
comprising the secondary station's respective communication
channel.
8. A secondary station as claimed in claim 7, characterised in that
the estimating means comprises means for resolving the signals
comprising its respective communication channel separately from
signals transmitted from primary station antennas not comprised in
its respective subset of antennas.
9. A secondary station as claimed in claim 7, characterised by
comprising at least as many antennas as the total number of primary
station antennas comprised in the mutually exclusive subsets which
transmit channels having no distinguishing features other than the
respective subset of antennas.
10. A secondary station as claimed in claim 7, characterised in
that means are provided for characterising at least one radio
channel using signals transmitted by the primary station and for
signalling the results of the characterisation to the primary
station.
11. A method of operating a radio communication system comprising a
primary station having at least two antennas and at least two
secondary stations each having at least one antenna, the method
comprising the primary station transmitting respective
communication channels between the primary station and first and
second secondary stations from respective mutually exclusive
subsets of the primary station's antennas, the first and second
secondary stations both being in the coverage area of the primary
station antennas in both subsets and the respective channels being
distinguished solely by the respective subsets of antennas, the
method further comprising each secondary station estimating from
the received signals the subset of signals comprising its
respective communication channel.
Description
[0001] The present invention relates to a radio communication
system and further relates to primary and secondary stations for
use in such a system and to a method of operating such a system.
While the present specification describes a system with particular
reference to the Universal Mobile Telecommunication System (UMTS),
it is to be understood that the apparatus and methods disclosed are
equally applicable to use in other mobile radio systems.
[0002] In a radio communication system, radio signals typically
travel from a transmitter to a receiver via a plurality of paths,
each involving reflections from one or more scatterers. Received
signals from the paths may interfere constructively or
destructively at the receiver (resulting in position-dependent
fading). Further, differing lengths of the paths, and hence the
time taken for a signal to travel from the transmitter to the
receiver, may cause inter-symbol interference.
[0003] It is well known that the above problems caused by multipath
propagation can be mitigated by the use of multiple antennas at the
receiver (receive diversity), which enables some or all of the
multiple paths to be resolved. For effective diversity it is
necessary that signals received by individual antennas have a low
cross-correlation. Typically this is ensured by separating the
antennas by a substantial fraction of a wavelength, although
closely-spaced antennas may also be employed by using techniques
disclosed in our International patent application WO 01/71843
(applicant's reference PHGB000033). By ensuring use of
substantially uncorrelated signals, the probability that
destructive interference will occur at more than one of the
antennas at any given time is minimised.
[0004] Similar improvements may also be achieved by the use of
multiple antennas at the transmitter (transmit diversity).
Diversity techniques may be generalised to the use of multiple
antennas at both transmitter and receiver, known as a Multi-Input
Multi-Output (MIMO) system, which can further increase system gain
over a one-sided diversity arrangement. As a further development,
the presence of multiple antennas enables spatial multiplexing,
whereby a data stream for transmission is split into a plurality of
sub-streams, each of which is sent via many different paths. One
example of such a system is described in U.S. Pat. No. 6,067,290,
another example, known as the BLAST system, is described in the
paper "V-BLAST: an architecture for realising very high data rates
over the rich-scattering wireless channel" by P W Wolniansky et al
in the published papers of the 1998 URSI International Symposium on
Signals, Systems and Electronics, Pisa, Italy, Sep. 29 to Oct. 2,
1998.
[0005] The performance gains which may be achieved from a MIMO
system may be used to increase the total data rate at a given error
rate, or to reduce the error rate for a given data rate, or some
combination of the two. A MIMO system can also be controlled to
reduce the total transmitted energy or power for a given data rate
and error rate.
[0006] One area in which MIMO techniques may be applied is a
High-Speed Downlink Packet Access (HSDPA) scheme, which is
currently being developed for UMTS and which may facilitate
transfer of packet data to a mobile station at up to 4 Mbps. In one
proposed embodiment of HSDPA separate data streams are sent from
respective antennas at a Base Station (BS), which data streams can
in principle be received and decoded by a Mobile Station (MS)
having at least as many antennas as there are data streams.
[0007] A problem with the use of a MIMO system for packet data
transmission is the impact of differing radio link qualities on the
communication system. For example, some of the data streams may
have very poor quality radio links, and if all the data is combined
this will degrade the performance of the other links and reduce
overall system capacity.
[0008] An object of the present invention is to provide a radio
communication system having improved performance.
[0009] According to a first aspect of the present invention there
is provided a radio communication system comprising a primary
station having at least two antennas and at least two secondary
stations each having at least one antenna, wherein the primary
station comprises means for transmitting respective communication
channels between the primary station and first and second secondary
stations from respective mutually exclusive subsets of the primary
station's antennas, wherein the first and second secondary stations
are in the coverage area of the primary station antennas in both
subsets, wherein the respective channels are distinguished solely
by the respective subsets of antennas, and wherein the first and
second secondary stations each comprise means for estimating from
the received signals the subset of signals comprising its
respective communication channel.
[0010] By utilising different antennas on the primary station to
transmit to different secondary stations, system capacity can be
improved in situations where signals received by a secondary
station from some of the primary station's antennas are of poor
quality. In contrast to known beam-forming or space-division
multiple access (SDMA) techniques, which use phased-arrays of
antenna elements at the primary station to generate beams in the
direction of particular secondary stations, the present invention
requires no angular separation between secondary stations receiving
signals from different primary station antennas.
[0011] The present invention enables a radio communication system
that normally operates as a MIMO system to improve its system
capacity in certain situations by using different base station
antennas to address different secondary stations rather than having
all the antennas address one secondary station. In a UMTS
embodiment, simultaneous transmissions to different secondary
stations from different antennas can be made using the same
channelisation code, scrambling code, carrier frequency and
timeslots, thereby improving the overall spectral efficiency of the
system.
[0012] According to a second aspect of the present invention there
is provided a primary station having at least two antennas for use
in a radio communication system having at least two secondary
stations, wherein means are provided for transmitting respective
communication channels between the primary station and first and
second secondary stations from respective mutually exclusive
subsets of the primary station's antennas, wherein the first and
second secondary stations are in the coverage area of the primary
station antennas in both subsets, and wherein the respective
channels are distinguished solely by the respective subsets of
antennas.
[0013] According to a third aspect of the present invention there
is provided a secondary station having at least one antenna for use
in a radio communication system having respective communication
channels between a primary station having at least two antennas,
the secondary station and a further secondary station, wherein the
communication channels are transmitted from respective mutually
exclusive subsets of the primary station's antennas, wherein the
secondary station and the further secondary station are in the
coverage area of the primary station antennas in both subsets,
wherein the respective channels are distinguished solely by the
respective subsets of antennas, and wherein means are provided for
estimating from the received signals the subset of signals
comprising the secondary station's respective communication
channel.
[0014] The secondary station may estimate the signals corresponding
to its respective communication channel by treating signals from
other subsets of the primary station's antennas, intended for other
secondary stations, as noise. However, in a preferred embodiment
the secondary station comprises means for resolving the signals
comprising its respective communication channels separately from
signals transmitted from primary station antennas not comprised in
its respective subset of antennas.
[0015] According to a fourth aspect of the present invention there
is provided a method of operating a radio communication system
comprising a primary station having at least two antennas and at
least two secondary stations each having at least one antenna, the
method comprising the primary station transmitting respective
communication channels between the primary station and first and
second secondary stations from respective mutually exclusive
subsets of the primary station's antennas, the first and second
secondary stations both being in the coverage area of the primary
station antennas in both subsets and the respective channels being
distinguished solely by the respective subsets of antennas, the
method further comprising each secondary station estimating from
the received signals the subset of signals comprising its
respective communication channel.
[0016] Embodiments of the present invention will now be described,
by way of example, with reference to the accompanying drawings,
wherein:
[0017] FIG. 1 is a block schematic diagram of an embodiment of a
MIMO radio system;
[0018] FIG. 2 is a block schematic diagram of an embodiment of a
base station for a MIMO radio system which weights sub-stream
signals before transmission;
[0019] FIG. 3 is a graph showing the variation in channel capacity
between two stations each having two antennas as the relative gain
of the transmitting antennas is altered;
[0020] FIG. 4 is a block schematic diagram of an embodiment of a
radio system made in accordance with the present invention, in
which different sub-streams are directed at different terminals;
and
[0021] FIG. 5 is a flow chart illustrating operation of a radio
communication system made in accordance with the present
invention.
[0022] In the drawings the same reference numerals have been used
to indicate corresponding features.
[0023] FIG. 1 shows an example of a MIMO system for the
transmission of downlink packet data from a primary station 100 to
a secondary station 110. The primary station 100 comprises a data
source 102 which provides a data stream for transmission to the
secondary station 110. This stream is divided by a serial to
parallel converter (S/P) 104 to generate a plurality of data
sub-streams which are provided to a transmitter (TX) 106. The
transmitter 106 arranges for the data sub-streams to be sent to
multiple antennas 108 (labelled 1, 2, 3 and 4 in FIG. 1) for
transmission from the Base Station (BS) 100 to a Mobile Station
(MS) 110. The antennas 108 are assumed to be substantially
omni-directional (or designed to give coverage over a sectored
cell).
[0024] Suitable coding, typically including Forward Error
Correction (FEC), may be applied by the BS 100 before serial to
parallel conversion. This is known as vertical coding, and has the
advantage that coding is applied across all sub-streams. However,
problems may arise in extracting the sub-streams since joint
decoding is needed and it is difficult to extract each sub-stream
individually. As an alternative each sub-stream may be coded
separately, a technique known as horizontal coding which may
simplify receiver operation. These techniques are discussed for
example in the paper "Effects of Iterative Detection and Decoding
on the Performance of BLAST" by X Li et al in the Proceedings of
the IEEE Globecom 2000 Conference, San Francisco, Nov. 27 to Dec.
1, 2000.
[0025] If vertical coding is used the FEC which is applied must
have sufficient error-correcting ability to cope with the entire
MIMO channel, which comprises a plurality of paths. It will be
appreciated that the set of paths between BS 100 and MS 110 will
typically include direct paths and indirect paths, the latter being
where signals are reflected by one or more scatterers.
[0026] The MS 110 comprises a plurality of antennas 118 (labelled
A, B, C and D in FIG. 1). Signals received by the antennas 118 are
provided to a receiver (RX) 116, which extracts the plurality of
transmitted data sub-streams from the received signals. The data
sub-streams are then recombined by a parallel to serial converter
(P/S) 114 and provided to a data output block 112. Although both
the BS 100 and MS 110 are shown as having the same number of
antennas, this is not necessary in practice and the numbers of
antennas can be optimised depending on space and capacity
constraints.
[0027] In the simplest implementation of a BS 100, each data
sub-stream is mapped to a separate antenna 108. Such an
implementation is appropriate for spatially uncorrelated radio
channels. In the general case, for which a suitable BS 100 is
illustrated in FIG. 2, each data sub-stream could be sent to each
antenna 108 after applying a complex weight 202 (with one weight
value per antenna 108 for each data sub-stream). This approach can
be used to map each data sub-stream to a different antenna beam.
The antenna beams may be aimed in predetermined directions, or the
directions may be determined dynamically to take advantage of
changing radio channel conditions. An example of a MIMO system with
dynamically changing beam directions is disclosed in our co-pending
unpublished United Kingdom patent application 0102316.7
(Applicant's reference PHGB010012). A special case of interest is
where each data stream is mapped to a subset of the antennas (i.e.
some of the weights are zero).
[0028] For simplicity, the following embodiments use the simplest
case of a one-to-one mapping between data sub-streams and antennas
108, but it will be appreciated that the present invention is not
limited to such a scenario.
[0029] The capacity improvements promised by a MIMO arrangement
have been shown to be achievable in a Gaussian channel even in an
open-loop arrangement, where the transmitter (BS 100) has no
knowledge of the downlink channel properties. Some capacity
improvement may also be achieved in an open-loop arrangement in a
Rayleigh fading channel, although the improvement is reduced if the
difference between the received Signal to Interference and Noise
Ratios (SINRs) from each transmitter antenna 108 (after
receive-diversity combining at the MS 110) is large.
[0030] A closed-loop scheme may provide additional benefits. The
amplitude of the transmissions from the different BS antennas 108
may be weighted so that the transmitter power is directed to the
antennas 108 which have the best received SINR at the MS 110. When
the BS antennas 108 are being used in a space-time coded MIMO
system, with different data being transmitted from each antenna,
then the data rate from each antenna may be adjusted as well as the
power, as disclosed for example in our co-pending unpublished
International patent application PCT/EP01/13690 (Applicant's
reference PHGB 000168). In the following discussion, it is assumed
that the total combined transmit power from all antennas is
limited.
[0031] In some situations, especially where the difference in
channel gain from the different transmit antennas is very large
(for example when one or more antennas are in a deep fade), the
best radio link capacity may be achieved by switching off one or
more of the BS antennas 108 altogether (i.e. reducing power or data
rate to zero). Such a strategy of closed-loop antenna selection
diversity transmission also has other advantages, such as reducing
the number of antennas required at the MS 110.
[0032] FIG. 3 is a graph showing a rudimentary comparison between
the available capacity C as a function of the channel gain G.sub.2
for the second transmitter antenna 108 for a given value of channel
gain G.sub.1 for the first transmitter antenna. The graph shows
capacities achievable using an open-loop space-time coded MIMO
system 302 and a closed-loop transmitter antenna selection scheme
304 for the case of two transmitter antennas 108 at the BS 100 and
two receiver antennas 118 at the MS 110. Note that the channel gain
with the antenna selection scheme is calculated after
receive-diversity combining has been performed at the MS 110.
[0033] It was assumed in the derivation of FIG. 3 that the total
available transmit power is a constant, P.sub.tx. Thus when
G.sub.2=G.sub.1, in the open-loop MIMO case P.sub.tx/2 is
transmitted from each of the BS antennas 108, but in the
antenna-selection case P.sub.tx is transmitted from one antenna and
no power from the other antenna. Note that the choice of
transmitter antenna 108 in the antenna-selection case is arbitrary
when G.sub.2=G.sub.1, and the capacity C.sub.1 in this situation is
greater than C.sub.2 due to the MIMO gain. However, when G.sub.2=0
the capacity in the open-loop MIMO case is much reduced to C.sub.3
as half of the transmitter power is being transmitted from a
useless antenna 108, whereas the capacity remains constant in the
closed-loop antenna-selection case.
[0034] Thus it can be seen from FIG. 3 that in the region marked
"X" it is beneficial to use closed-loop antenna-selection rather
than an open-loop MIMO scheme.
[0035] In known MIMO systems, downlink signals to one MS 110 in a
cell are distinguished from those for other MSs 110 by means of
frequency (Frequency Division Multiple Access, FDMA) and/or
timeslot (Time Division Multiple Access, TDMA) and/or code (Code
Division Multiple Access, CDMA), thereby defining the channel for
that MS 110. For a given MS, all available BS transmit antennas 108
are then used on the channel for that MS 110. However, in the case
described above, where the optimal transmission scheme would be one
of transmit antenna selection, the capacity of the system is
sub-optimal as certain antennas are not useful for transmission on
certain channels.
[0036] Hence, in a system made in accordance with the present
invention, a channel for a particular MS 110 may also be defined in
whole or in part by the subset of BS antennas 108 from which the
channel is transmitted, where the subset of BS antennas 108 may
consist of one or more antennas. Such a scheme may be termed
"Antenna-Division Multiple Access" (ADMA).
[0037] A simple example of an embodiment of the present invention
is illustrated in FIG. 4, comprising a BS 100 having two antennas
108 and two MSs 110a, 110b, each comprising two antennas 118. The
BS 100 comprises two data sources 102 (D1 and D2), each providing a
data sub-stream intended for a different MS 110 to a transmitter
(TX) 106. A practical system employing the present invention is
likely to include a larger number of MSs and larger numbers of
antennas.
[0038] Initial operation of such a system will now be described
with reference to the flow chart shown in FIG. 5. The operation
starts, at step 502, with the BS 100 signalling to the first MS
110a that there is data for transmission. The BS 100 then, at step
504, transmits signals from each of its antennas 108 to enable the
first MS 110a to characterise the radio channel. Such signals could
for example comprise orthogonal sequences of known pilot
information transmitted from each BS antenna 108. The MS 110a
measures, at step 506, the SINR of the respective received signals
(after diversity combining of the signals received from each
antenna 118). In this example, the first MS 110a thereby determines
that the SINR of the signal received from antenna 1 of the BS 110
is greater than the SINR of the signal received from Antenna 2 of
the BS.
[0039] After the measurements have been completed, the MS 110a
sends, at step 508, a signalling message to the BS 100 to indicate
that antenna 1 gives the best SINR. This message could take a
number of forms, for example:
[0040] the identity of the best and/or worst BS antenna(s) 108;
[0041] absolute SINR measurements for the best and/or worst BS
antenna(s) 108; or
[0042] absolute SINR measurements for each BS antenna 108.
[0043] As a result of this signalling message, the BS determines
that data destined for the first MS 110a should be transmitted only
from antenna 1. Antenna 2 of the BS 100 may therefore be used to
transmit data destined for a different MS, using the same
frequency, timeslots and channelisation codes as are being used
from antenna 1 for the first MS 110a. A second MS 110b is therefore
identified, at step 510, using the same protocol as described
above, for which antenna 2 gives the best SINR. Signals are then
transmitted to the first MS 110a from antenna 1 and to the second
MS 110b from antenna 2 at step 512. These transmission routes are
indicated by the solid arrows in FIG. 4, the dashed arrows
indicating that signals intended for one MS 110a, 110b may reach
the other MS since both mobiles are in the coverage area of both
antennas 108.
[0044] If each MS 110a, 110b has at least as many antennas as the
BS 100 and the radio channel contains sufficient scatterers to
cause the transfer function of the channel to be substantially
different between each BS transmit antenna 108 and each receive
antenna 118 of a particular MS 110, then each MS can decode the
signal from its corresponding BS antenna 108 without the signals
from the other BS antennas causing undue interference.
[0045] For example, in the scenario just described, the signals
received at the two antennas 108 of the first MS 110a are given by
1 ( r 1 r 2 ) = ( h 11 h 21 h 12 h 22 ) ( t 1 t 2 )
[0046] where r.sub.i is the signal received at the i.sup.th MS
antenna 118, t.sub.i is the signal transmitted by the i.sup.th BS
antenna 108, h.sub.ij is the complex channel transfer
characteristic from the i.sup.th BS antenna 108 to the j.sup.th MS
antenna 118.
[0047] t.sub.i corresponds to the signal transmitted from BS
antenna 1, which is the wanted signal for the first MS 110a. This
signal can therefore be extracted, in a channel extraction (CX)
block 414, as: 2 t 1 = h 22 r 1 - h 21 r 2 h 11 h 22 - h 12 h
21
[0048] The channel transfer coefficients h.sub.ij are determined
using known pilot information, as discussed above. The frequency
with which the pilot information needs to be transmitted and the
channel transfer coefficients updated will depend on the coherence
time of the channel. The known pilot information should be
transmitted with enough energy for a sufficiently accurate
estimation of the channel transfer coefficients to be obtained in
the presence of noise. The energy of the known pilot information
can be increased by transmitting a longer pilot information
sequence or by increasing the transmission power of the pilot
information. The length of each sequence of known pilot information
on which each estimate of channel transfer coefficients is based
should be significantly less than the coherence time of the
channel. Other known techniques, such as joint detection and
interference cancellation, may also be used by the MS 110 to
extract the desired signal from the signals received from the BS
antennas 108.
[0049] In practice, the received signals r.sub.1 and r.sub.2 will
also include noise terms. The transmitted signals can then be
recovered via a range of known methods, for example Minimum Mean
Square Error (MMSE) or Maximum Likelihood Sequence Estimation
(MLSE), as described for example in Chapter 16 of "Antennas and
Propagation for Wireless Communication Systems" by S R Saunders,
published by John Wiley and Sons in 1999. MMSE may be used as part
of the MLSE process, or may be used on its own. In the latter case,
no a priori knowledge is assumed about the possible sequences of
transmitted bits.
[0050] In a practical system, with a larger number of MSs 110, it
is unlikely to be desirable to use ADMA as the only means of
distinguishing channels intended for different MSs, as this would
require every MS 110 to have at least as many antennas 118 as the
total number of MSs in a cell. A typical scheme would therefore be
to use ADMA as an additional means of optimising the use of the
radio resources in conjunction with another multiple access scheme
such as CDMA, FDMA or TDMA. ADMA could then be used on a given
code/frequency/timeslot to distinguish channels to n different MSs,
where n is equal to the smallest number of antennas on any of the
MSs using that combination of code, frequency and timeslot. For
example, a pair of MSs each with two antennas could use ADMA to
share a downlink channel defined by a particular frequency and/or
timeslot and/or code.
[0051] In some embodiments of the present invention, a MS 110 may
not require as many antennas as suggested above. In the example of
a pair of MSs above, if signal from one BS antenna 108 is so weak
that it can be considered as noise, only one antenna 118 would be
required. However, this requires particular channel conditions and
so may not always be relied on. Our co-pending unpublished United
Kingdom patent application 0115937.5 (applicant's reference PHGB
010100) discloses a MIMO system in which a MS 110 may have fewer
antennas than the number of sub-streams directed at it. Such a
system employs sampling techniques to generate sufficient
substantially uncorrelated received signal samples to enable the
sub-streams to be extracted. One particularly favoured embodiment
of this system employs code division techniques to transmit the
signals and a Rake receiver in the MS 110 to determine the received
signal samples. Such techniques could also be applied to the
present invention, thereby relaxing the requirements on the number
of MS antennas 118.
[0052] A system made in accordance with the present invention
provides increased downlink capacity over a conventional system by
using all the available BS antennas 108 in an optimal way. In a
practical system, it is clearly likely that the best BS antennas
108 for each of a number of MSs may sometimes coincide. In this
case, a MMSE approach may be taken across the MS space to allocate
antennas 108 to the MSs 110 in a way which is optimal for the
system as a whole.
[0053] Embodiments of the present invention may make use of a
single antenna structure having a dual polarisation capability,
which can be used so as to be functionally equivalent to two
antennas. Other structures, for example printed circuit board
metallisation, may be used to provide functionality equivalent to
an antenna.
[0054] MIMO techniques for transmission to individual MSs 110 may
also be used in a system made in accordance with the present
invention, as it is still possible to assign a plurality of BS
antennas 108, being a subset of the total number of BS antennas, to
a particular MS 110. Further, the use of ADMA for at least one data
channel to each of a plurality of MSs 110 does not preclude the
simultaneous transmission of one or more other data channels to one
or more of the same MSs 110 by means of other multiple access
techniques, for example code division or frequency division.
[0055] In the above description, the term `Base Station` or
`Primary Station` relates to an entity which may in practice be
distributed between a variety of parts of the fixed infrastructure.
In a UMTS system, for example the functions of a BS 100 are carried
out in a "Node B", which is the part of the fixed infrastructure
directly interfacing with a MS 110, and at a higher level in the
Radio Network Controller (RNC). As well as their use in
transmission of data from a BS 100 to a plurality of MSs 110, the
techniques described may also be used in the reverse direction (for
example during UMTS soft handover). In this case, the roles of the
BS 100 and MS 110 would be reversed in the description above, with
the BS 100 adopting the role of a secondary station and the MS 110
the role of a primary station.
[0056] From reading the present disclosure, other modifications
will be apparent to persons skilled in the art. Such modifications
may involve other features which are already known in the design,
manufacture and use of radio communication systems and component
parts thereof, and which may be used instead of or in addition to
features already described herein.
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